The present disclosure relates to a method for measuring the maturity or cell wall thickening of a sample of cellulosic fiber. The method at least includes exposing the sample of fiber to polarized light, capturing one or more images of the sample through crossed polar lenses and a compensator plate so that the image(s) include interference colors from the sample; and conducting computer analysis on the captured image(s) to determine the maturity or degree of cell wall thickening of the cellulosic fiber by comparing the image(s) to reference color interference data.